GB2237387A

GB2237387A - Coulometric titration system

Info

Publication number: GB2237387A
Application number: GB8921621A
Authority: GB
Inventors: George Robertson; Steven Moore
Original assignee: PETROCHEM ANALYSIS Ltd; CSC SCIENT Co Ltd
Current assignee: PETROCHEM ANALYSIS Ltd; CSC SCIENT Co Ltd
Priority date: 1989-09-25
Filing date: 1989-09-25
Publication date: 1991-05-01
Anticipated expiration: 2009-09-25
Also published as: GB8921621D0; GB2237387B

Abstract

A titration system comprises reaction cell 1 containing detection electrodes 2 and electrolysis electrodes 3 immersed in a solution 4 of the substance to be analysed, and a constant current generator 13 which supplies a constant electrolysis current at one of a plurality of levels. An electrolysis current monitor 14 generates a signal S3 when the constant current level fluctuates or becomes unstable, and signal S3 causes the microprocessor 9 to change the level of the constant current signal. A detection means, 6, 7, 8 coupled to the detection electrodes 2 generates a signal corresponding to the polarization of the solution 4 which may also cause alteration of the constant current. The microprocessor 9 includes a counter for integrating the current required for the electrolysis until an end point is reached. The titration system maintains a constant current level and integrates at an appropriate rate even if increases in electrical resistance in the solution prevent maintenance of a higher level of constant current. The system may be used to determine water in crude oil. <IMAGE>

Description

TITRATION METHODS AND DEVICES This invention relates to titration methods and devices of the coulometric type and in particular, but not exclusively, to titration methods and devices which are suitable for the determination of the quantity of water in crude oil samples.

Coulometric titrators have been available for a long time, examples of such devices being described in US patent specifications 3 726 778 (G V Levy et al) and 4 211 614 (Eppstein et al).

These prior art titration devices comprise electrolytic electrodes which are immersed in a soluton of a substance to be analysed. Detection electrodes are also immersed in the solution for monitoring the polarization potential of the solution. The polarization potential, this being indicative of the conductivity of the solution, indicates the amount of water present in the solution.

In the devices of the US patent specifications, pulsed currents are applied to the electrolytic electrodes for generating iodine in the solution by electrolysis. The free iodine reacts with water to reduce the concentration of water in the solution. The titration end point is reached as soon as a minute excess of free iodine is present in the solution.

The quantity of electrolytic current supplied to the electrolytic electrodes for the duration of the titration is indicative of the quantity of iodine produced in the solution. This in turn corresponds to the quantity of water in the original sample.

In the titration devices of the US patent specifications, the electric current is supplied to the electrolytic electrodes in a pulsed manner at a constant level of typically 400 mA for progressively shorter durations until the end point of the titration. These devices suffer from the disadvantage that a relatively high current is supplied to the electrolytic electrodes at the end point of the titration. This leads to an over-estimate in the quantity of iodine necessary to perform the titration.

These titration devices fail to produce accurate results especially when used for measuring water content in samples of crude oil.

When using the titration devices for measuring the quantity of water in a sample of crude oil, the sample is mixed with a solvent, such as xylene or toluene. The solvent improves the solubility of the crude oil sample and inhibits the accumulation of waxy deposits on the titration apparatus. The addition of the solvent to the crude oil sample and the oil itself tend to alter the electrical resistance of the solution particularly when relatively high proportions of solvent (ie 20% to 30%) are used. The resistance of the solution can become sufficiently high to lower the level of current between the electrolytic electrodes. In consequence, less iodine is generated in the solution than expected.

While this is happening, the operator may be under the false impression that the titration rate is being conducted at a higher level than it actually is. A lower level of current gives rise to a lower production rate of iodine giving the false impression that a greater quantity of water is present in the crude oil sample than is in fact the case.

It is an aim of the present invention to eliminate or at least alleviate the aforementioned problem.

According to the present invention, there is provided a titration device comprising: electrolytic electrodes for immersion into a solution of a substance to be analysed; a constant current generator for supplying a constant current at one of a plurality of levels between the electrolytic electrodes for electrolysing the solution; detection electrodes for immersion into the solution; detection means coupled to the detection electrodes for generating a signal corresponding to the polarization potential of the solution; integration means for integrating the electrolytic current required for the electrolysis until an end point is reached; a current monitor, responsive to fluctuations in the constant current supplied between the electrolytic electrodes, for generating a signal when the level of the constant current falls or becomes unstable; and means for changing the constant current at said one level to a different constant current level in response to receipt of said signal when the constant current at said one level can no longer be maintained.

The changing means is preferably operative for changing the constant current at said one level to a different constant current level in dependence upon changes in the polarization potential of the solution.

Embodiments of the invention have the advantage that they can respond to changes, in particular increases, in the electrical resistance of the solution during titration thereby ensuring that the titration rate is being conducted at a level appropriate for the current being supplied to the electrolytic electrodes.

In a preferred embodiment, the current monitor may comprise a comparator for comparing the voltage across the electrolytic electrodes with a reference voltage.

The signal may be generated by the comparator when the voltage falls below a predetermined level, thereby indicating that a constant current supply can no longer be maintained.

A microprocessor may be included in the changing means for receiving the signal and for generating one or more switching signals corresponding to a lower level of constant current to be supplied to the electrolytic electrodes.

The changing means may include a plurality of transistors, each switching signal switching on a different transistor so that a control signal, corresponding to the lower level of the constant current, is generated by the changing means. In this case, the constant current generator generates a constant current which corresponds to the level of the control signal.

In embodiments of the invention, each constant current level may correspond to a different titration count rate. The integration means is operative for counting the total number of titration counts until the end point is reached. For example, there may be five different constant current levels, these corresponding to count rates of 1, 4, 8, 16 and 32 counts per second from the lowest to the highest current level. These may respectively correspond to titration rates of 1. 4, 8, 16 and 32 microgramms of water per second.

More or fewer than five different current levels may be adopted in accordance with the working requirements of the device.

The device may be provided with a liquid crystal display for displaying the characteristics, for example mass, of the substance analysed. A status indicator, in the form of a liquid crystal display may also be provided for providing a graphic representation of the polarization voltage.

Embodiments of the invention may be provided with an interface for communicating output data with an external computer. A keyboard may be provided for entering additional manipulative data into the device. A printer may be provided for producing a hard copy of data generated by the device.

According to the present invention there is also provided a method of titration comprising: immersing electrolytic electrodes into a solution of a substance to be analysed, supplying a constant current at one of a plurality of levels between the electrolytic electrodes for electrolysing the solution; immersing detection electrodes into the solution; detecting the polarization potential of the solution; monitoring the constant current supplied between the electrolytic electrodes; and changing the constant current at said one level supplied to the electrolytic electrodes to a different constant current level when the constant current at said one level can no longer be maintained.

The monitoring of the constant current supplied between the electrolytic electrodes may be carried out by comparing a voltage corresponding to the voltage across the electrolytic electrodes with a reference voltage and determining that the constant current can no longer be maintained when the voltage falls below a predetermined level.

Embodiments of the invention have the advantage that in the event that the constant current supply becomes unstable owing to an increased solution resistance or battery power depletion (in the case of a battery powered device), a lower constant current level can be supplied to the electrolytic electrodes in order that the current flowing between the electrolytic electrodes accurately corresponds to the expected titration rate.

The invention will now be further described by way of example, with reference to the accompanying drawings, in which; Figure 1 is a block diagram of a titration device embodying the present invention; Figure 2 is a circuit diagram of part of the embodiment of Figure 1; and Figure 3 shows graphs for the purpose of illustrating operation of the embodiment of Figure 1.

The block diagram of Figure 1 shows an electrolytic reaction cell 1 containing detection electrodes 2 and electrolytic electrodes 3 which are immersed in a solution 4 of a substance to be analysed. The solution 4 may be, for example, a sample of crude oil and water mixed with xylene or toluene for helping to disperse the oil within the solution. Naturally, other kinds of solvents may be used, for example methanol, depending upon the type of substance to be analysed.

In this embodiment, the coulometric titration method involves the production of iodine in situ, by electrolysis. Water present in the solution will react with the iodine as it is generated. The quantity of iodine, and hence electric charge supplied to the electrolytic electrodes 3, required to react with all of the water present in the solution is indicative of the quantity of water initially present in the solution.

A detector voltage generator 5 is coupled to the detector electrodes 2 for passing a constant alternating current across the detection electrodes 2 for generating a voltage signal corresponding to the polarization potential of the solution 4. The polarization potential is amplified by an amplifier 6 before being converted into D.C. level form by an A.C. to D.C. converter 7.

The D.C. level signal represents the polarization potential of the solution 4 and is fed to a voltage comparator 8 which generates a voltage signal having one of five levels corresponding to five different ranges of polarization potential.

An electric current i of one of five possible levels is supplied to the electrolytic electrodes 3. Which of the five current levels is supplied is partly determined according to the level of the polarization potential.

The five voltage signals produced by the voltage comparator are Vaa Vb, Vc, Vd and V . The e voltage signal Va is generated by the voltage comparator 8 when the polarization potential of the solution 4 is below a "base line" voltage of 190 mV.

The polarization voltage of this level corresponds to too much iodine in the solution. This can be corrected by adding water to the solution to bring it up to "base line". When the polarization voltage is between 190 and 220 mV, the voltage comparator 8 supplies a voltage signal Vb corresponding to the "base line. The "base line" corresponds to the state of the solution when no titration is taking place. The polarization potential rises above 220 mV as water is present in the solution 4. When the polarization voltage is between 220 and 300 mV, a voltage signal Vc is generated by the voltage comparator 8, this corresponding to the lowest of three different constant current levels. This first level is 10 mA and corresponds to a tritation count rate of one count per second.A tritation count rate of one count per second corresponds to one microgram of water per second.

When the polarization voltage is between 300 and 500 mV, the second constant current level of 85 mA is supplied to the electrolytic electrodes, this corresponding to a titration count rate of eight counts per second. At this level, a voltage signal Vd is supplied by the voltage comparator 8. A fifth and highest constant current level of 342 mA is supplied to the electrolytic electrodes 3 when the polarization potential is greater than 500 mV. This constant current level corresponds to a titration rate of 32 counts per second and is represented by an output voltage signal Ve from the voltage comparator 8.

One of three different constant current levels is therefore supplied to the electrolytic electrodes 3 depending on which of voltage signals Vc to Ve is fed to the microprocessor 9.

The microprocessor 9 includes a counter for counting the titration count rate corresponding to the constant current level supplied to the electrolytic electrode 3 for the time duration of the titration. The end point of the titration is defined by sufficient electric charge being supplied to the electrolytic electrode 3 so that enough iodine has been generated to react with all of the water present in the solution. The total count counted by the microprocessor 9 corresponds to the number of micrograms of water in the original sample.

This information can be displayed on a liquid crystal display 10 in addition to other operational information, for example, "ready", "start", "wait" (ie before start of next filtration) etc.

A status indicator 11, in the form of a liquid crystal display, provides a graphic representation of the polarization potential of the solution 4 as a function of time. A drift calculation/subtraction unit 12 is coupled to the microprocessor 9 for compensating for the effects of moisture in the air on the titration.

The constant current i supplied to the electrolytic electrodes 3 is generated by a constant current generator 13. When the microprocessor 9 receives one of the voltage signals V to V , it generates one or c e more switching signals or a first signal S1 which is fed to a current range selector 15. The first signal switches the current range selector 15 to deliver a control signal or second signals S2 having one of three predetermined values corresponding to the three voltages Vc to Ve The second signal S2 is received by the constant current generator 13, which adjusts the constant current i to a level of 10 mA; 85 mA or 342 mA as the case may be.

A current monitor 14 is responsive to fluctuations in the constant current i produced by the constant current generator 13. In the event that the electrical resistance of the solution 4 increases to a high level owing to, for example, the presence of the solvent xylene and oil in the sample, the constant current generator 13 may no longer be able to deliver the constant current level i which corresponds to the appropriate voltage signal Vc to Ve supplied by the voltage comparator 8. If this happens, iodine is being generated in the solution 4 at a lower rate than the titration rate being counted by the microprocessor 9 in response to the appropriate level of voltage signal.

As soon as the current monitor 14 detects instability or a fall in the level of the constant current signal i, a third signal S3 is supplied to the microprocessor 9.

Receipt of the third signal 53 causes the microprocessor 9 to modify the first signal S1. The modified first signal S1 causes the second signal S2 supplied by the current range selector 15 to change so that the constant current generator 13 supplies a lower constant current i to the electrolytic electrodes 3.

Since a lower constant current i is being supplied to the electrolytic electrode 3, stability will be achieved even though the resistance of the solution 4 has increased. The microprocessor 9 changes the titration count rate to the level corresponding to the new constant current i supplied to the electrolytic electrode 3.

The second signal S2 supplied by the current range selector 15 can have up to five different levels altogether. Three of these correspond to the three constant current levels 10 mA, 85 mA and 342 mA and the two other levels correspond to two intermediate constant current levels and corresponding titration count rates.

For example, if the polarization potential of the solution 4 is high so that the voltage comparator 8 supplies a voltage signal Ve to the microprocessor 9, the values of the first signal S1 and the second signals are such that the constant current generator 13 supplies highest constant current of 342 mA. If the resistance of the solution becomes high and this level of constant current of 342 mA cannot be maintained, this fact is detected by the current monitor 14. The third signal S3 generated by the current monitor 14 is received by the microprocessor unit 9. which responds by supplying a modified first signal S1 to the current range selector 15. This modified first signal S1 corresponds to a lower constant current level. This lower current level is between that corresponding to the voltage signals Vd (85 mA) and Ve (342 mA).In this case, the lower constant current is about 200 mA and corresponds to 16 counts per second.

Similarly, in the event that the resistance of the solution increases to such an extent that a constant current level of 85 mA (corresponding to a titration rate of 8 counts per second) cannot be maintained, the third signal 53 is again supplied to the microprocessor 9. The first and second signals S1 and are therefore modified again so that the constant current generator 13 supplies a lower current of, say 50 mA. This lower current corresponds to d titration count rate of 4 counts per second.

Although the titration will take a little longer owing to a lower constant current level being supplied, the microprocessor 9 will always count at a rate which corresponds to the actual quantity of iodine being generated by the electrolytic electrodes 3. The microprocessor 9 can be programmed to supply the highest constant current level which fulfills the aforementioned conditions so as to achieve the highest count rate possible while maintaining an accurate count rate which corresponds to the actual quantity of iodine produced.

The operation of the constant current generator 13, the current monitor 14 and the current range selector will be described in greater detail later with reference to Figure 2.

The microprocessor unit 9 may be coupled to a standard serial interface (RS 232 output) 16 for communicating output data with an external computer capable of conducting further processing operations on the titration data.

A keyboard 17 may be coupled to the microprocessor unit 9 for permitting the entry of data into the device. For example, the microprocessor unit 9 may be programmable so that different constant current levels and/or different titration rates may be assigned to different constant current levels depending upon the materials used in the solution.

A calculator 18 may be provided in association with the keyboard and microprocessor unit 9 for manipulating data fed into the microprocessor 9 by the keyboard 17. A printer 19 may be provided for producing a hard copy of titration data.

The apparatus of the constant current generator 13 the current monitor 14 and the current range selector 15 will be described in more detail below with reference to Figures 2 and 3.

Figure 2 is a circuit diagram of the constant current generator 13, current monitor 14 and the current range selector 15. The constant current generator 13 is of standard construction and comprises an operational amplifier IC 1 having positive and negative inputs. The output of the IC 1 is coupled to the base electrode of a transistor T6 via a resistor R9. The collector and base electrodes of the transistor T6 are coupled together via a capacitor C1. A collector terminal 20 of the transistor T6 is connected to one of the electrodes of the electrolytic electrodes 3 while the other electrode is coupled to a voltage of 50 to 70 volts at a terminal 21. The level of the constant current supplied by the constant current generator 13 depends upon the value of the second signal S2 supplied to the positive terminal of the operational amplifier IC 1.

The current range selector 15 is capable of generating up to five different values for the second signal and consequently can cause the constant current to generator 13 to supply up to five different constant current levels. The current range selector 15 is connected between a ground voltage and +5 volts. A resistor R1 and zener diode ZD1 generate a reference voltage V ref of 4 volts which feeds a potential divider network comprising: resistors R2 + R3; resistors R4 to R8: and variable resistors VR1 to VR5. The microprocessor 9 is operative for delivering the first signal S1 to one or more of the gate electrodes of transistors T2 to T5. In this embodiment the transistors T2 to T5 are FETs.The supply of the first signal S1 to the transistor (also an FET) T1 switches the signal S2 off. The combination of switching of the transistors T2 to T5 depends upon the desired level of constant current to be supplied by the constant current generator 13. The microprocessor 9 supplies the first signal S1 to the gate electrodes of the transistors T2 to T5 which give rise to an appropriate level of the second signal S2 which in turn causes the constant current generator 13 to supply the desired constant current level. Table 1 below is a truth table showing the five different levels of the second signal which can be supplied to the operational amplifier IC 1 according to various switching sequences of the transistors T2 to T5.

TABLE 1 Voltage (V) T2 T3 T4 T5 APProximate of Second Constant Sianal S2 Current (mA) SuPPlied 3.427 off off off off 342 1.7136 on off off off 171 0.8570 on on off off 85 0.4284 on on on off 43 0.01071 on on on on 11 Pull-up resistors R17 to R21 are provided for each of the gate electrodes. The variable resistors VR1 to VR5 can be adjusted so as to vary the values of S2 and in consequence, to vary the constant current levels to be supplied by the constant current generator 13.

The current monitor 14 comprises a potential divider made up of resistors R10, R11, R12 and variable resistor VR6. This potential divider is connected between the collector electrode of the transistor T6 and ground. A second operational amplifier IC 2 receives a reference voltage at its positive input and a voltage level from the potential divider at its negative input. The operational amplifier IC2 provides the third signal S3 when the difference between the positive and negative inputs of the operational amplifier IC2 exceeds a predetermined level. The signal S3 is fed to the microprocessor unit 9 whereupon the sequence of switching of transistors T2 to T5 is changed in order that a new appropriate current level can be supplied by the constant current generator 13.

As the resistance of the solution 4 increases, the voltage across the electrolytic electrodes 3 increases in order that a constant current can be supplied between the electrodes 3. As the resistance increases further, the voltage supply cannot continue to rise and so the constant current level begins to fall. In consequence, the potential difference across the potential divider R10 to R12 falls thus giving rise to a change in the difference between the positive and negative inputs of the operational amplifier IC2. The characteristics of the operational amplifier IC2 are chosen so that when the voltage of the collector electrode of the transistor T6 falls to about 2 volts, the third signal S3 rises to a significant level which is detected by the microprocessor unit 9 as being an instruction to switch the constant current level and corresponding titration count rate to a lower value.

Graph I of Figure 3 illustrates the profile of the polarization voltage as a function of time during the titration process. Graph II illustrates the constant current level supplied to the electrolytic electrodes -3 during normal titration cell conditions which do not encounter unacceptably high increases in solution resistance. In this case, the initial constant current level supplied to the electrolytic electrodes 3 is high (342 mA) and the microprocessor unit counts at a titration rate of 32 counts per second. As the quantity of water in the solution falls, the polarization potential falls so the voltage comparator 8 delivers a voltage signal corresponding to Vd to the microprocessor unit 9.In response to this, the microprocessor unit 9 delivers an appropriate first signal S1 to the current range selector 15 which switches the transistors T2 to T5 in such a manner that the second signal S2 causes the constant current generator 13 to supply a lower constant current level of 85 mA to the electrolytic electrode 3. The microprocessor unit 9 then counts at a titration count rate of 8 counts per second for the duration that this constant current level of 85 mA is supplied.

When the polarization potential of the solution 4 falls further owing to the reaction of water contained in the solution with the iodine generated, the voltage comparator 8 produces the voltage signal Vc corresponding to the count rate of 1 count per second and a constant current level of lOmA.

Graph III of Figure 3 illustrates the case where an intermediate constant current level corresponding to 16 counts per second is adopted. In this case, the microprocessor unit 9 initially supplies the voltage signal Ve corresponding to the constant current level of 352mA since the water content of the initial solution is high. However, in this case the resistence of the solution is sifficiently high to prevent a constant current level of the correct amount of 342 mA from being supplied by the constant current generator 13, the current monitor 14 responds by generating the third signal 53 The microprocessor unit 9 responds to the third signal S3 by switching the transistors T2 to T5 of the current range selector 15 so that the intermediate constant current level of 200mA is supplied to the electroytic electrodes 3. The microprocessor unit 9 counts at the corresponding titration count rate of 16 counts per second. The resistance of this solution does not change greatly after this point and so the remaining titration process proceeds normally.

It is to be understood that modifications may be made to embodiments of the invention without departing from the scope of the invention. For example, a different number of current levels or intermediate current levels may be adopted depending on the circumstances.

Claims

1. According to the present invention, there is provided a titration device comprising: electrolytic electrodes for immersion into a solution of a substance to be analysed; a constant current generator for supplying a constant current at one of a plurality of levels between the electrolytic electrodes for electrolysing the solution; detection electrodes for immersion into the solution: detection means coupled to the detection electrodes for generating a signal corresponding to the polarization potential of the solution; integration means for integrating the electrolytic current required for the electrolysis until an end point is reached; a current monitor, responsive to fluctuations in the constant current supplied between the electrolytic electrodes, for generating a signal when the level of the constant current falls or becomes unstable; and means is provided for changing the constant current at said one level to a different constant current level in response to receipt of said signal when the constant current at said one level can no longer be maintained.

2. A titration device according to Claim 1, wherein the changing means is operative for changing the constant current at said one level to a different constant current level also in dependence upon changes in the polarization potential of the solution.

3. A titration device according to Claim 1 or Claim 2, wherein the current monitor comprises a comparator for comparing the voltage across the electrolytic electrodes with a reference voltage, the comparator generating the signal when the voltage falls below a predetermined level, thereby indicating that the constant current supply can no longer be maintained.

4. A titration device according to Claim 3, wherein the changing means includes a microprocessor for receiving the signal and for generating one or more switching signals corresponding to a lower level of constant current to be supplied to the electrolytic electrodes.

5. A titration device according to Claim 4. wherein the changing means includes a plurality of transistors, and each of said one or more switching signals switches on a different transistor so that a control signal, corresponding to the lower level of the constant current, is generated by the changing means, the constant current generator generating a different constant current corresponding to the level of the control signal.

6. A method of titration comprising: immersing electrolytic electrodes into a solution of a substance to be analysed, supplying a constant current at one of a plurality of levels between the electrolytic electrodes for electrolysing the solution; immersing detection electrodes into the solution; detecting the polarization potential of the solution; monitoring the constant current supplied between the electrolytic electrodes: and changing the constant current at said one level supplied to the electrolytic electrodes to a different constant current level when the constant current at said one level can no longer be maintained.

7. A method according to Claim 6. wherein the step of monitoring the constant current comprises comparing the voltage across the electrolytic electrodes with a reference voltage, and determining that the constant current can no longer be maintained when voltage falls below a predetermined level.

8. A titration device substantially as hereinbefore described with reference to Figure 1 or Figures 1 and 2 of the accompanying drawings.

9. A method of titration according to Claim 6 substantially as hereinbefore described.